FIG. 10 shows, in cross-section, a distributed feedback type (hereinafter referred to as DFB) semiconductor laser device as disclosed in Electronics Letters, Volume 18, Number 23, page 1006 (1982) by Y. Itaya et al. In FIG. 10, reference numeral 1 designates an n type InP substrate. N type InP cladding layer 11 is disposed on the substrate 1. N type InGaAsP active layer 5 is disposed on the cladding layer 11. P type InGaAsP diffraction grating layer 2' is disposed on the active layer 5. P type InP cladding layer 6 is disposed on the diffraction grating layer 2'. P.sup.+ type InGaAsP contact layer 7 is disposed on the cladding layer 6. P side electrode 8 is disposed on the contact layer 7 and n side electrode 9 is disposed on the rear surface of the substrate 1. Reference numeral 3 designates a diffraction grating produced by varying the layer thickness of the diffraction grating layer 2' to produce a concavo-convex surface on that layer 2' adjacent the cladding layer 6.
When a forward direction bias is applied between the p side electrode 8 and n side electrode 9, holes are injected from the p side electrode 8 and electrons are injected from the n side electrode 9, and they recombine at the active layer 5 to generate light. This device has a waveguide structure in which the active layer 5 and the diffraction grating layer 2', both having relatively large refractive indices, are disposed between n type InP cladding layer 11 and p type InP cladding layer 6 having relatively low refractive indices. The emitted light is transmitted in the active layer 5, the diffraction grating layer 2', and the neighborhood thereof in a direction parallel to those layers. Furthermore, since the diffraction grating 3 is produced on the diffraction grating layer 2', there is a periodic variation of effective refractive index in the direction of the diffraction grating 3. If the period of the diffraction grating 3 is made equal to the period at which the generated light undergoes Bragg reflection, only light having a wavelength that satisfies the Bragg reflection condition is repeatedly reflected in the waveguide structure and produces oscillations.
FIG. 11 shows a cross-sectional view of a phase-shift DFB semiconductor laser device as disclosed in Electronics Letters, Volume 20, Number 24, pages 1016-1018 (1984) by H. Soda et al, and FIG. 12 shows a perspective view thereof. In this figure, reference numeral 5' designates an InGaAsP active layer, reference numeral 10 designates an anti-reflection film, reference numeral 12 designates an n type InGaAsP guiding layer, and reference numerals 13 and 14 designate a p type and n type InP buried layer, respectively.
When a forward bias is applied between the p side electrode 8 and n side electrode 9, holes are injected from the p side electrode 8 and electrons are injected from the n side electrode 9, and they recombine at the active layer 5' to generate light. This device has a waveguide structure in which the active layer 5' and the guiding layer 12, both having relatively large refractive indices, are disposed in the layer thickness direction, the n type InP substrate 1 and the p type InP cladding layer 6 both having relatively low refractive indices. The active layer 5' is disposed between the p type InP cladding layers 13 or n type Inp cladding layers 14 in the transverse direction. The emitted light is transmitted in the active layer 5, the guiding layer 12, and the neighborhood thereof in the stripe direction. Furthermore, since the diffraction grating 3 is disposed on the substrate 1, there is a periodic variation in the thickness of the guiding layer 12. Therefore, there is a periodic variation in the equivalent refractive index. If the period of the diffraction grating 3 is made equal to the period at which the generated light is Bragg reflected, only the light of a wavelength that satisfies the Bragg reflection condition is repeatedly reflected in the waveguide structure and produces oscillations. However, a semiconductor laser device having such a constant period diffraction grating does not actually oscillate at the Bragg wavelength. It oscillates at two wavelengths shifted slightly toward the long and short wavelength side, respectively, from the Bragg wavelength. In order to oscillate at a single wavelength, it is sufficient to shift the phase of the light by .pi./2 upon its return to a central portion of the element after reflection. In this prior art example, as shown by FIG. 12, the width of the central portion of stripe having length l is widened. By widening the stripe width, the equivalent refractive index of that portion is differentiated from that of the other portion. When the variation quantity .DELTA..beta. of the propagation constant is selected so that .DELTA..beta.l=.pi./b 2, the light is shifted by .pi./2 while being transitting the broad width stripe portion, thereby producing a single wavelength oscillation.
FIG. 14(c) shows, in cross-section, a .lambda./4 shifted DFB semiconductor laser as disclosed in Electronics Letters, Volume 20, Number 24, pages 1008-1010, by K. Utaka et al. FIGS. 14(a) and 14(b) show a production process and FIG. 14(c) shows a completed device. In these figures, reference numeral 1 designates an n type InP substrate. A negative photoresist film 16 is disposed on the substrate 1. Reference numeral 28 designates a .lambda./4 shift position. A positive photoresist film 25 is disposed on the negative photoresist film 16. Reference numerals 17 and 18 designate a light exposed portion and non-exposed portion, respectively, of the photoresist film 25. Reference numeral 2' designates an n type InGaAsP diffraction grating layer and reference numeral 5' designates an InGaAsP active layer. Reference numeral 12 designates a p type InGaAsP guiding layer and reference numeral 6 designates a p type InP cladding layer. Reference numeral 20 designates a p type InP first buried layer, reference numeral 21 designates an n type InP second buried layer, and reference numeral 22 designates a p type InP third buried layer. Reference numeral 7 designates a p.sup.+ type InGaAsP contact layer, reference numeral 24 designates a silicon dioxide film, and reference numeral 30 designates a zinc diffusion region. Reference numerals 8 and 9 designate a p side electrode and an n side electrode, respectively.
A negative photoresist 16 is deposited on the n type InP substrate 1, and the photoresist 16 at the left side of the .lambda./4 shift position 28 is removed. A positive photoresist 25 is deposited on the entire surface and exposed with light interference fringes to produce alternating exposed portions 17 and non-exposed portions 18. First, development of the positive photoresist 25 is carried out. The positive photoresist 25, after development, is used as an etching mask to etch the n type InP substrate 1. Thereafter, the positive photoresist 25 is removed and development of the negative photoresist 16 is carried out. The left side of the .lambda./4 shift position 28 is covered by other photoresist and the n type InP substrate 1 at the right side of the .lambda./4 shift position 28 is etched using the negative side photoresist 16 as an etching mask. Since the positive photoresist and the negative photoresist have inverted photosensitive characteristics, a diffraction grating having inverted phases to the left and right of the .lambda./4 shift position 28 is obtained.
Next, an n type InGaAsP diffraction grating layer 2', an InGaAsP active layer 5', a p type InGaAsP guiding layer 12, and a p type InP cladding layer 6 are grown. The neighborhoods of both facets are buried by the p type InP first buried layer 20, the n type InP second buried layer 21, and the p type InP third buried layer 22. The p.sup.+ type InGaAsP contact layer 7 is grown on the entire surface. Finally, an SiO.sub.2 film 24 for confining the current is produced and zinc 30 is diffused to reduce the contact resistance. A p side electrode 8 and an n side electrode 9 are vapor deposited to complete the laser element.
When a forward direction bias is applied between the p side electrode 8 and the n side electrode 9, holes are injected from the p side electrode 8 and electrons are injected from the n side electrode 9 and recombine at the active layer 5' to generate light. Since this device has a waveguide structure in which the active layer 5', the diffraction grating layer 2', and the guiding layer 12, all relatively having large refractive indices, are disposed between the n type Inp substrate 1 and the p type InP cladding layer 6, both having relatively low refractive indices, the emitted light transits the active layer 5', the diffraction grating layer 2', the guiding layer 12, and the neighborhood thereof in the stripe direction. Furthermore, since the thickness of the diffraction grating layer 2' varies periodically, there is also a periodic variation in the equivalent refractive index. If the period of the diffraction grating 3 is made equal to the period at which the generated light undergoes Bragg reflection, only the light of a wavelength that satisfies the Bragg reflection condition is repeatedly reflected in the waveguide structure. Further, the phase of the diffraction grating 3 is inverted at the center of the resonator and, thus, a single wavelength oscillation occurs. The buried layer at the laser facet prevents reflections.
In the prior art DFB semiconductor laser device of FIG. 10, the diffraction grating is usually produced by etching after growing a cladding layer 6, an active layer 5, and a diffraction grating layer 2' on the semiconductor substrate 1. Thus, the distance between the diffraction grating and the active layer depends on the depth of etching. When a cladding layer is regrown on the diffraction grating, the diffraction grating is partly dissolved, i.e., melted back, which reduces the amplitude of the diffraction grating. The melting back makes control of the coupling coefficient, i.e., the degree of the distributed feedback, difficult.
In the prior art phase shift DFB semiconductor laser device of FIGS. 11 and 12, in order to obtain desired characteristics, the stripe width of the active layer has to be precisely controlled and difficulties in achieving this control produced low yields.
Furthermore, in the prior art DFB semiconductor laser device of FIG. 10, the coupling coefficient representing the intensity of the distributed feedback is uniform in the resonator direction. The light intensity distribution in the resonator is increased in the neighborhood of the center as shown by A in FIG. 13. Therefore, there arises a so-called axial direction spatial hole burning at high power output operation, causing an instability in the mode of oscillation and preventing the achievement of narrow line widths.
Further, in the prior art .lambda./4 shift DFB laser device of the above-described construction, in order to produce a diffraction grating having an inverted phase, the interference fringe exposure requires the use of both negative and positive resists. In order to produce diffraction gratings on the left and right of the .lambda./4 shift position, an etching step, in which depth and the etched configuration are difficult to control and which can affect laser element characteristics, must be carried out.